function custom_nav_menu() { ?>
    your custom menu
<?php }

function place_menu() {
    if (is_single() && in_category(array('5','6','7')) || is_category(array('5','6','7'))) {
        add_action('thesis_hook_after_header', 'custom_nav_menu');
    } else {
        remove_action('thesis_hook_before_header','thesis_nav_menu');
        add_action('thesis_hook_after_header','thesis_nav_menu');
    }
}
add_action('thesis_hook_before_html','place_menu');

http://diythemes.com/forums/thesis-customization/4512-custom-menus-if-cat-two-menus-if-else-one-menu.html#post23140

Rearrange Thesis nav

wordpress

PHP